1. Field of the Invention
This invention relates to a data transmission system and more particularly, but not by way of limitation, it relates to a data transmission system by way of a telephone line suitable for a pay-TV system.
2. Description of the Prior Art
A pay-TV system has been developed to eliminate troublesome commercial messages or possible degradation in broadcasting quality due to various restrictions involved in free commercial broadcasting. In particular, a wireless pay-TV system has a great future because it can curtail expenses and time required for laying cables.
FIG. 1 is a block diagram of a basic system formation of a wireless pay-TV system. Generally, in the system, since a voluntary program is provided for pay, a television signal is scrambled by an encoder at a station side and received after decoded through a decoder at a subscriber's side and a charge is paid depending on a received amount. In FIG. 1, a program source 1 is contents of a program and an encoder/transmitter 2 scrambles a television signal and transmits the scrambled signal through a transmitting antenna AT1. A decoder 3 decodes the scrambled signal received through a receiving antenna AT2 and outputs to an ordinary TV set 6. On the other hand, the decoder is connected to a converter 4 and a computer system 5 at a transmitting side generally through a telephone line for transmitting data such as data for charges.
FIG. 2 illustrates in detail a basic formation diagram of the decoder illustrated in FIG. 1. A TV signal processor 31 receives the scrambled television signal and decodes it under control of a decode controller 32 to output a decoded signal to the TV set 6. The decode controller 32 controls a TV signal processor 31 and makes the data of charges for programs the subscriber has viewed be stored at a memory 33. An automatic calling apparatus 34 automatically calls the computer system 5 through a transmission line 8 generally formed of a telephone line having a telephone exchanger 7 connected intermediate thereof to establish a data transmission line. A data transmitter 35 transmits the data of charges stored at the memory 33 to the computer system 5 through the transmission line 8 and then receives from the computer system a key code required for decoding the scrambled television signal for storing the code at the memory.
In a wireless pay-TV system, generally, a video signal and an aural signal are radiated into space in the form of radio waves, while transmission of data for charges etc. is carried out according to necessity by way of the telephone line. FIG. 3 is a block diagram of a conventional data transmission system using a telephone line. As soon as the system of FIG. 3 is automatically or manually put into starting conditions when data transmission is required, an automatic calling apparatus 92 produces a timing signal for starting an operation and lets a line monitor 93 detects the line is occupied by a telephone set 12. When the line monitor 93 detects that the line is not occupied by the telephone set 12, the automatic calling apparatus 92 catches the line 10 and automatically transmits a preliminarily selected signal (a tone signal or a dial impulse signal corresponding to a telephone number of a central equipment 11).
The central equipment 11 is comprised of a signal receiver/line changer 111 and a data transmitter 112 automatically responds to a calling signal coming from the line based on the calling from a subscriber's equipment 9 to establish a transmission line 10 with the subscriber's equipment 9. Then, data transmission is carried out between a data transmitter 91 of the subscriber's equipment 9 and the data transmitter of the central equipment 11 through the transmission line 10.
Since in the known method, the subscriber's equipment is caused to automatically start, when data transmission is necessitated, to automatically call the central equipment 11 for effecting data transmission, the automatic calling apparatus 92 and the line monitor 93 are required for the system and accordingly the manufacturing cost of the system must be increased. Furthermore, because of its automatic operation, a control circuit should be provided in the subscriber's equipment for assuring a proper operation where the central equipment 11 is busy or where "0" transmission is required for a private branch exchanger used in the system, and accordingly the subscriber's equipment must be complicated.